1. Field of the Invention
The present invention relates to a photomask for focus monitoring, a method of focus monitoring, a unit for focus monitoring and a manufacturing method of the electronic device.
2. Description of the Background Art
Increases in the integration and the miniaturization in semiconductor integrated circuits have been remarkable in recent years. Together with that, the miniaturization of the circuit pattern formed on a semiconductor substrate (hereinafter referred to simply as a wafer) has greatly progressed.
In particular, photolithographic technology is widely recognized as a basic technology in the pattern formation. Accordingly, a variety of developments and improvements have been carried out up to the present time. However, the miniaturization of patterns shows no signs of slowing down and demand for increase in resolution of the patterns is on the increase.
Such a photolithographic technology is a technology for transcribing patterns from a photomask (original image) to a photoresist applied on a wafer so that an etched film in the lower layer is patterned by using this transcribed photoresist.
At this time of photoresist transcription, a development treatment is carried out on the photoresist and a photoresist wherein the portion hit by light through this development treatment is removed is called a positive type while a photoresist wherein the portion hit by light is not removed is called a negative type photoresist.
In general, the resolution limit R (nm) in photolithographic technology using a downscaling exposure method is represented as:
R=k1xc2x7xcex/(NA)
Here, xcex is the wavelength (nm) of the utilized light, NA is the numerical aperture in the projection optical lens system and k1 is a constant depending on the image formation condition and the resist process.
As is seen from the above equation, there is a method of making the values of k1 and xcex smaller and of making the value of NA larger in order to achieve an increase in the resolution limit R, that is to say, to gain microscopic patterns. That is to say, in addition to making the constant, which depends on the resist process, smaller, a shortening of the wavelength and an increase of NA may be implemented.
From among these, a shortening of the wavelength of the light source is technically difficult and, therefore, it becomes necessary to increase the NA for the same wavelength. When an increase in NA is implemented, however, the focal depth xcex4(xcex4=k2xc2x7xcex/(NA)2) of light becomes shallow and, therefore, there are problems such that deterioration in form and in dimension precision of formed patterns is caused.
In order to expose a photoresist according to the patterns of a photomask with a high resolution using such photolithographic technology, it is necessary to carry out the exposure under the condition wherein the photoresist accords with the optimal image formation surface (optimal focus surface) of the projection optical system within the range of the focal depth. Therefore, it is necessary to precisely find the distance from the surface of the exposed substrate to the projection optical system. The process of finding this distance is called focus monitoring.
Concerning conventional focus monitoring, there is, for example, the method of phase shift focus monitoring developed by Brunner of IBM Corporation and sold by Benchmark Technology Corporation of the United States and the phase shift focus monitoring mask that is used in this method.
FIG. 56 is a view for describing the operational principle of the method of phase shift focus monitoring. In reference to FIG. 56, a phase shift focus monitoring mask 105 is used in this method of phase shift focus monitoring. This phase shift focus monitoring mask 105 has a transparent substrate 105a, a light blocking film 105b having a predetermined pattern and a phase shifter 105c that is formed on this predetermined pattern.
Concretely, this phase shift focus monitoring mask 105 has a pattern wherein a thin light blocking pattern 105b is arranged between sufficiently thick transmission portions 105d and 105e, as shown in FIG. 57. Here, a phase shifter 105c is not placed in transmission portion 105d while a phase shifter 105c is placed on transmission portion 105e. 
In reference to FIG. 56, in this method of phase shift focus monitoring, first, phase shift focus monitoring mask 105 is irradiated with light. At this time, since phase shifter 105c is formed so as to shift the phase of the transmission light by approximately 90xc2x0, in the case that the light that has passed through transmission portion 105e precedes the light that has passed through the transmission portion 105d by the optical path difference of 1/4 xcex, 5/4 xcex . . . , or in the case that the light that has passed through transmission portion 105e succeeds the light that has passed through the transmission portion 105d by the optical path difference of 3/4 xcex, 7/4 718 xcex . . . , the light acts in a mutually reinforcing manner. Thereby, the light after passing through phase shift focus monitoring mask 105 has an asymmetric intensity distribution with respect to the z axis (optical axis). This light that has passed through phase shift focus monitoring mask 105 is condensed by means of projection lens 119a and 119b so as to form an image on a photoresist 121b, which is on a semiconductor substrate 121a. 
According to this method of phase shift focus monitoring, an image is formed on photoresist 121b under the condition wherein the intensity distribution of the diffracted light is asymmetric with respect to the z axis (optical axis: the longitudinal direction in the figure). Therefore, an image of the pattern shifts in the direction (x-y direction: lateral direction in the figure) perpendicular to the z axis (optical axis) on wafer 121 due to the shift of wafer 121 in the z direction. By measuring this amount of shift of the image of the pattern in the x-y direction, the measurement of the position in the z direction, that is to say the measurement of the focus, becomes possible.
In addition to the above described method of phase shift focus monitoring there is a method disclosed in, for example, Japanese Patent Laying-Open No. 6-120116(1994) that is a method of focus monitoring. In this method, a first predetermined pattern in the photomask surface is first irradiated with an exposure light of which the main light beam has the first angle of inclination and, thereby, the first image of the first predetermined pattern is exposed on a substrate of photosensitive material. After that, a second predetermined pattern that is different from the above first predetermined pattern is irradiated with an exposure light of which the main light beam has a second angle of inclination that differs from the first angle of inclination and, thereby, the second image of the second predetermined pattern is exposed on the substrate of the photosensitive material. By measuring the distance between the exposed first and second images, the distance from the position of the substrate of the photosensitive material to the optimal image formation surface can be found from the relationship between this distance and the amount of defocus.
In this method, a predetermined pattern on the photomask surface is irradiated according to the first angle of inclination or according to the second angle of inclination and, therefore, a photomask 205 having the structure as shown in FIG. 58 is used.
In reference to FIG. 58, this photomask 205 has a transparent substrate 205a, marks for position measurement 205b1, and 205b2 formed on the surface of this transparent substrate 205a and a diffraction grid pattern 205c formed on the rear surface of transparent substrate 205a. That is to say, an exposure light that has struck photomask 205 is diffracted by diffraction grid pattern 205c so that mark for position measurement 205b1 is irradiated according to the first angle of inclination and mark for position measurement 205b2 is irradiated according to the second angle of inclination.
In the above described phase shift focusing monitor, however, it is necessary to use a phase shift mask of a specific structure as photomask 105. There is a problem point that the photomask becomes expensive because a photomask of such a specific structure is necessary.
In addition, in a conventional method of phase shift focus monitoring, it is necessary to use illumination which is isotropic (pupil plane is circular) and of which the angle spread is small, that is to say that has a small "sgr" value, in order to gain a high detection sensitivity in the z direction (ratio of the of shift amount in the x-y direction to the shift amount in the z direction). This is described in T. A. Brunner et al., xe2x80x9cSimulations and experiments with the phase shift focus monitor,xe2x80x9d SPIE Vol. 2726, pp. 236-243. In particular, FIG. 4 of the above reference shows that when the "sgr" value is 0.3, the shift amount in the x-y direction of the pattern (focus monitor overlay error) becomes of the maximum and the detection sensitivity in the z direction becomes high.
It is necessary to reduce the diameter of aperture 14d of illumination aperture unit 14, such as the illumination diaphram as shown in, for example, FIG. 59.
However, when the device pattern is formed by using illumination of which the "sgr" value is small, such as approximately 0.3, the coherence of the light is too intense and transformation of the secondary pattern transcribed to the photoresist becomes significant. In order to prevent such transformation of the secondary pattern, it is necessary to make the "sgr" value be, for example, 0.6 or higher, by making the diameter of the aperture of the illumination aperture unit 14 used at the time device pattern formation greater than the diameter of the aperture of the illumination aperture unit 14 used at the time of focus monitoring. Therefore, illumination aperture unit 14 must be replaced at the time between focus monitoring and device pattern formation and, therefore, there is a problem that labor and maintenance become necessary for the replacement.
In addition, since at the time of replacement the lenses become clouded in the case that an oxygen mixture remains in the illumination optical system, it is necessary to carry out oxygen purging by introducing nitrogen for a long period of time after the replacement and, therefore, there is a problem such that the operation becomes complicated.
In addition, in a method disclosed in the Japanese Patent Laying-Open No. 6-120116(1994), it is necessary to form a diffraction grid pattern 205c on the rear surface of a photomask 205 as shown in FIG. 58. It is necessary for this diffraction grid pattern 205c to be a microscopic pattern so as to allow light to diffract. There is a problem point that the process dimensions become small because of the necessity for forming such a microscopic pattern and the fabrication of the photomask becomes difficult.
In addition, it is necessary to illuminate only the portion of the rear surface of photomask 205 where diffraction grid pattern 205c exists with an exposure light and, therefore, there is also a problem that the illumination range must be concentrated in one small portion.
A purpose of the invention is to provide a photomask for focus monitoring, a method of focus monitoring, a unit for focus monitoring and a manufacturing method for the electronic device, wherein the fabrication of the photomask is easy and the replacement of the illumination aperture unit is unnecessary while focus monitoring of which the detection sensitivity in the z direction is high becomes possible.
A photomask for focus monitoring of the present invention is a photomask for focus monitoring that is used for focus monitoring wherein a position in an optical system of the exposed surface is measured in order to adjust the focus of the optical image on the exposed surface at the time of pattern exposure and is provided with a substrate that allows the exposure light to pass through and unit mask structures for focus monitoring. A unit mask structure for focus monitoring is provided with two patterns for position measurement and a light blocking film. The two patterns for position measurement are formed on the surface of the substrate in order to measure the relationship between the mutual positions. The light blocking film is formed on the rear surface of the substrate and has a rear surface pattern for substantially differentiating the incident directions of the exposure light that enters the two patterns for position measurement. When the dimension of the rear surface pattern is L and the wavelength of the exposure light is xcex, L/xcex is 10 or higher.
According to the photomask for focus monitoring of the present invention, the dimension L of the rear surface pattern is determined so that L/xcex becomes 10 or higher, and, therefore, the dimensions of the pattern can be sufficiently enlarged to the degree such that diffraction can be ignored. Since the process dimensions of the rear surface pattern become enlarged in such a manner, the fabrication of the photomask for focus monitoring becomes easy.
In addition, by sufficiently differentiating the incident directions of the exposure light that enters the two patterns for position measurement through the provision of such a rear surface pattern, a high detection sensitivity in the z direction, of which the degree is approximately the same as that of the method of phase shift focus monitoring wherein the "sgr" value is reduced using the conventional illumination, can be gained. In addition, it is not necessary to reduce the a value as in the phase shift focusing monitoring method and, therefore, it is not necessary to replace the illumination aperture unit at the time between focus monitoring and device pattern formation.
In the above described photomask for focus monitoring, the rear surface pattern formed in the light blocking film, on the rear surface of the substrate, is preferably formed so as to block a portion the exposure light that enters, at least, either of the two patterns for position measurement and so as to allow only the remaining exposure light to enter in the case that the light blocking film is not formed.
Thereby, the incident directions of the exposure light that enters the two patterns for position measurement can be made to differ substantially by providing a light blocking film having patterns.
In the above described photomask for focus monitoring, the rear surface pattern preferably consists of a group of patterns that are shared by the two patterns for position measurement.
Thereby, it is not necessary to provide the rear surface patterns in the same number as the number of patterns for position measurement so that the number of rear surface patterns can be reduced.
In the above described photomask for focus monitoring, the rear surface pattern preferably consists of a pattern that is symmetrically arranged with respect to a point of the rear surface of the substrate facing the central point between the two patterns for position measurement.
Thereby, design of the rear surface pattern becomes easy.
In the above described photomask for focus monitoring, the above described pattern symmetrically arranged-with respect to the point on the rear surface of the substrate is preferably arranged symmetrically with respect to the second fictitious line gained by projecting the bisector perpendicular to the first fictitious line connecting two patterns for position measurement to the rear surface of the substrate.
Thereby, telecentricity of the exposure light that irradiates the two patterns for position measurement can be made to be symmetric.
In the above described photomask for focus monitoring, the rear surface pattern is preferably an aperture pattern that is substantially circular.
The incident direction of the exposure light that enters the two patterns for position measurement can be made to differ substantially even when such a circular aperture pattern is used.
In the above described photomask for focus monitoring, when the aperture diameter of the circular aperture pattern that becomes the rear surface pattern is r, the depth of the substrate is D, the numerical aperture is NA and the coherence that is the coherency index of the exposure light is "sgr", the value of sin(tanxe2x88x921(r/D)) is preferably smaller than the INA value (=NAxc3x97"sgr"/projection magnification) that is the amount of spread of illumination.
In the case that the value of sin(tanxe2x88x921(r/D)) is the INA value or higher, a portion of the exposure light that enters the patterns for position measurement cannot be blocked depending on the arrangement position of the rear surface pattern and, thereby, the incident directions of the exposure light that enters the two patterns for position measurement cannot be made to differ.
In the above described photomask for focus monitoring, when the aperture diameter of the circular aperture pattern that becomes the rear surface pattern is r, the depth of the substrate is D, the numerical aperture is NA and the coherence that is the coherency index of the exposure light is xcex1, the value of sin(tanxe2x88x921(r/D)) is greater than the value of the INA value (=NAxc3x97"sgr"/projection magnification), which is the amount of spread of illumination, multiplied by 0.1.
In the case that the value of sin(tanxe2x88x921(r/e)) is the value of the INA value multiplied by 0.1 or less, the amount of the exposure light becomes {fraction (1/100)} of the case of the conventional transcription or less, and it becomes difficult to transcribe the patterns for position measurement to the photosensitive material. As a result, the throughput of the measurement of the focus is lowered.
In the above described photomask for focus monitoring, the rear surface pattern is preferably a pattern that allows a light blocking film to remain in a substantially a circular form.
The incident directions of the exposure light that enters the two patterns for position measurement can be made to differ substantially by using, in the above manner, the pattern that allows a circular light blocking film to remain.
In the above described photomask for focus monitoring, when the aperture diameter of the pattern that allows a circular light blocking film to remain that becomes the rear surface pattern is r, the depth of the substrate is D, the numerical aperture is NA and the coherence that is the coherency index of the exposure light is "sgr", the value of sin(tanxe2x88x921(r/D)) is preferably smaller than the INA value (=NAxc3x97"sgr"/projection magnification) that is the amount of spread of illumination.
In the case that the value of sin(tanxe2x88x921(r/D)) is the INA value or higher, a portion of the exposure light that enters the patterns for position measurement cannot be blocked depending on the arrangement position of the rear surface pattern and, thereby, the incident directions of the exposure light that enters the two patterns for position measurement cannot be made to differ.
In the above described photomask for focus monitoring, when the aperture diameter of the pattern that allows a circular light blocking film to remain which becomes the rear surface pattern is r, the depth of the substrate is D, the numerical aperture is NA and the coherence that is the coherency index of the exposure light is "sgr", the value of sin(tanxe2x88x921(r/D)) is preferably greater than the value of the INA value (=NAxc3x97"sgr"/projection magnification), which is the amount of spread of illumination, multiplied by 0.5.
In the case that the value of sin(tanxe2x88x921(r/D)) is the value of the INA value multiplied by 0.5 or less, the light blocking portion becomes small and it becomes difficult to secure the non-telecentric characteristics of the exposure light. As a result, the detection sensitivity of the pattern in focus monitoring is lowered.
Now, the meaning of non-telecentric characteristics is that, for example, the main light beam of the exposure light enters the exposed body with an inclination relative to the optical axis of the illumination optical system.
In the above described photomask for focus monitoring, the rear surface pattern is preferably a rectangular aperture pattern.
By using a rectangular aperture pattern in such a manner, the incident directions of the exposure light that enters the two patterns for position measurement can be made to differ substantially.
In the above described photomask for focus monitoring, when the thickness of the substrate is D, the numerical aperture is NA and the coherence, which is the interference indication of the exposed light, is "sgr", the part within the range where the distance from the sides of the rectangular aperture pattern, which is the rear surface pattern, is R or less, satisfying the equation sin(tanxe2x88x921(R/D))xe2x89xa7INA value (=NAxc3x97"sgr"/projection magnification), is preferably the light blocking part wherein a light blocking film is formed.
Thereby, only the illumination light from the rectangular aperture pattern can be made to enter each of the two patterns for position measurement so that the non-telecentric characteristics of the exposure light can be firmly secured.
In the above described photomask for focus monitoring, the rectangular aperture pattern is preferably a square aperture pattern.
In the case that a square aperture pattern is used in such a manner, the incident directions of the exposure light that enters the two patterns for position measurement can also be made to differ substantially.
In the above described photomask for focus monitoring, the two patterns for position measurement are preferably located, respectively, in the positions on the surface of the substrate that face the center points of the respective sides of the square aperture pattern facing each other.
Thereby, the two patterns for position measurement can be respectively illuminated with illumination light of which the distribution of the incident angle is half as in the case wherein the rear surface light blocking film, which is complementary, does not exist.
In the above described photomask for focus monitoring, when the length of a side of the square aperture pattern, which is the rear surface pattern, is a, the thickness of the substrate is D, the numerical aperture is NA and the coherence, which is the interference indication of the exposure light, is "sgr", the value of sin(tanxe2x88x921(a/D)) is preferably greater than a value twice the INA value (=NAxc3x97"sgr"/projection magnification), which is the amount of spread of the illumination.
Thereby, the entirety of the maximum incident angle components of the illumination on the side where the aperture exists can reach the patterns for position measurement through the above square aperture pattern.
In the above described photomask for focus monitoring, when the length of a side of the square aperture pattern, which is the rear surface pattern, is a, the thickness of the substrate is D, the numerical aperture is NA and the coherence, which is the interference indication of the exposure light, is "sgr", the value of sin(tanxe2x88x921(a/D)) is preferably smaller than a value three times the INA value (=NAxc3x97"sgr"/projection magnification), which is the amount of spread of the illumination.
In the case that the value of sin(tanxe2x88x921(a/D)) is three times greater or more, than the INA value, the square aperture pattern becomes too large and, thereby, the unit mask structure for focus monitoring becomes too large to arrange a plurality of unit mask structures, as described above, on the same mask or to carry out focus monitoring in a plurality of points within exposure field.
In the above described photomask for focus monitoring, when the length of a side of the rectangular aperture pattern, which is the rear surface pattern, is a, the thickness of the substrate is D, the numerical aperture is NA and the coherence, which is the interference indication of the exposure light, is "sgr", the value of sin(tanxe2x88x921(a/D)) is preferably greater than a value 0.2 times the INA value (=NAxc3x97"sgr"/projection magnification), which is the amount of spread of the illumination.
When the value of sin(tanxe2x88x921(a/D)) is 0.2 times the INA value or less, the amount of the exposure light becomes {fraction (1/100)} or less of the case of a conventional transcription and it becomes difficult to transcribe the patterns for position measurement to the photosensitive material. As a result of this, the throughput of the measurement of the focus is lowered.
In the above described photomask for focus monitoring, the rear surface pattern is preferably a rectangular pattern that allows a light blocking film to remain.
In the case that a rectangular pattern that allows a light blocking film to remain is used in such a manner, the incident directions of the exposure light that enters the two patterns for position measurement can also be made to differ substantially.
In the above described photomask for focus monitoring, when the thickness of the substrate is D, the numerical aperture is NA and the coherence, which is the interference indication of the exposed light, is "sgr", the part within the range where the distance from the sides of the rectangular pattern that allows a light blocking film to remain, which is the rear surface pattern, is R, or less, satisfying the equation sin(tanxe2x88x921R/D))xe2x89xa7INA value (=NAxc3x97"sgr"/projection magnification), is preferably the light blocking part wherein a light blocking film is not provided.
Thereby, an aperture that allows the patterns for position measurement to be exposed with a sufficient amount of light can be created outside of the rectangular pattern that allows a light blocking film to remain.
In the above described photomask for focus monitoring, the rectangular pattern that allows a light blocking film to remain is preferably a square pattern that allows a light blocking film to remain.
In the case that a square pattern that allows a light blocking film to remain is used in such a manner, the incident directions of the exposure light that enters the two patterns for position measurement can also be made to differ substantially.
In the above described photomask for focus monitoring, the two patterns for position measurement are preferably located, respectively, in the positions on the surface of the substrate that face the center points of the sides, respectively facing each other, of the square pattern that allows a light blocking film to remain.
Thereby, the two patterns for position measurement can be respectively illuminated with illumination light of which the distribution of the incident angle is half as in the case wherein the rear surface light blocking film, which is complementary, does not exist.
In the above described photomask for focus monitoring, when the length of a side of the square pattern that allows a light blocking film to remain, which is the rear surface pattern, is a, the thickness of the substrate is D, the numerical aperture is NA and the coherence, which is the interference indication of the exposure light, is "sgr", the value of sin(tanxe2x88x921(a/D)) is preferably greater than a value twice the INA value (=NAxc3x97"sgr"/projection magnification), which is the amount of spread of the illumination.
Thereby, the incident angle components of the illumination on the side where the light blocking pattern exists can be completely blocked so as not to reach the patterns for position measurement.
In the above described photomask for focus monitoring, when the length of a side of the square pattern that allows a light blocking film to remain, which is the rear surface pattern, is a, the thickness of the substrate is D, the numerical aperture is NA and the coherence, which is the interference indication of the exposure light, is "sgr", the value of sin(tanxe2x88x921(a/D)) is preferably smaller than a value three times the INA value (=NAxc3x97"sgr"/projection magnification), which is the amount of spread of the illumination.
In the case that the value of sin(tanxe2x88x921(a/D)) is three times greater or more than the INA value, the square aperture pattern becomes too large and, thereby, the unit mask structure for focus monitoring becomes too large to arrange a plurality of unit mask structures, as described above, on the same mask or to carry out focus monitoring in a plurality of points within exposure field.
In the above described photomask for focus monitoring, when the length of a side of the rectangular pattern that allows a light blocking film to remain, which is the rear surface pattern, is a, the thickness of the substrate is D, the numerical aperture is NA and the coherence, which is the interference indication of the exposure light, is "sgr", the value of sin(tanxe2x88x921(a/D)) is preferably greater than a value 0.5 times the INA value (=NAxc3x97"sgr"/projection magnification), which is the amount of spread of the illumination.
When the value of sin(tanxe2x88x921(a/D)) is 0.5 times the INA value or less, the light blocking part becomes small and it becomes difficult to secure the non-telecentric characteristics of the exposure light. As a result of this, the pattern detection sensitivity in focus monitoring is lowered.
In the above described photomask for focus monitoring, one of the two patterns for position measurement is preferably an inner box pattern of a box-in-box type while the other is an outer box pattern.
Thereby, the positional shift of the patterns for position measurement in the x-y plane can be easily measured.
In the above described photomask for focus monitoring, a box edge of, at least, either the inner box pattern or of the outer box pattern is preferably formed of either the line pattern, the space pattern or a plurality of hole patterns arranged at constant intervals.
A variety of patterns can be used in such a manner for a box edge of the box pattern.
In the above described photomask for focus monitoring, when a constant depending on the resist process and on the image formation conditions is k1, the wavelength of the exposure light is xcex and the numerical aperture is NA, the size S of the pattern in the box edge of, at least, either the inner box pattern or the outer box pattern preferably satisfies S=k1xc3x97xcex/NA(0.3 less than ki less than 0.6).
Thereby, measurement of the focus corresponding to the actual device becomes possible.
In the above described photomask for focus monitoring, when the thickness of the substrate is D, the numerical aperture is NA and the coherence, which is the interference indication of the exposure light, is "sgr", the distance N between the centers of the two patterns for position measurement is preferably greater than 0.5 times and smaller than 4 times the product (=INA valuexc3x97D) of the INA value, which is the amount of spread of the illumination (=NAxc3x97"sgr"/projection magnification), and D.
Thereby, the patterns for position measurement can be appropriately irradiated. In the case that the distance M between the two patterns for position measurement is 0.5 times the value of the INA valuexc3x97D or less, neither of the two patterns for position measurement can be diagonally irradiated. In addition, making the distance M between the two patterns for position measurement four times the value of the INA valuexc3x97D or. greater does not yield substantial benefits and unnecessarily increases the dimensions of the unit mask structures for focus monitoring such that a large number of unit mask structures for focus monitoring cannot be arranged on the mask.
The above described photomask for focus monitoring is preferably further provided with a mask structure for correcting the wafer position shift amount that has two additional patterns for position measurement, formed on the surface of the substrate, for the measurement of the mutual positional relationship and a pattern formed on a light blocking film formed on the rear surface of the substrate for making substantially equal the incident directions of the exposure light that enters the above two additional patterns for position measurement.
It becomes possible to measure the shift amount of the amount of movement at the time of the carrying out of the second exposure after moving the exposed body, subsequent to the first exposure, by providing at least one mask structure for correcting the wafer position shift amount.
In the above described photomask for focus monitoring, mask structures for correcting the wafer position shift amount is preferably formed in two or more places.
It also becomes possible to measure the shift amount in the rotational direction of the exposed body by providing the mask structures for correcting the wafer position shift amount in two or more places.
In the above described photomask for focus monitoring, the maximum value of the arrangement distance between two arbitrary structures from among mask structures for correcting the wafer position shift amount that are formed in two or more places is preferably greater than xc2xd of the dimension in the longitudinal direction of the exposure region for one shot.
Thereby, in the case that the exposure region for one shot is exposed while being shifted in the rotational direction, the detection sensitivity of the shift amount in this rotational direction can be increased.
In the above described photomask for focus monitoring, a plurality of unit mask structures for focus monitoring are preferably formed on the substrate so that the pitch between the two neighboring unit mask structures for focus monitoring is no less than 8 mm and no more than 20 mm.
Thereby, at the required spatial distance it becomes possible to measure the distance between the optical system and the surface of the substrate.
The method of focus monitoring according to the present invention is a method of focus monitoring used in focus monitoring for measuring the positions of the exposed surface in the optical system in order to adjust the focus of the optical image vis-à-vis the exposed surface at the time of the exposure of the pattern and focus monitoring is carried out by utilizing the characteristics that the image formed on the surface of photosensitive material of the photomask pattern is moved in the direction perpendicular to the optical axis when it moves in the optical axis direction on the surface of the photosensitive material by irradiating the photomask for focus monitoring with the exposure light. The photomask for focus monitoring is provided with a substrate for the exposure light to pass through and the unit mask structures for focus monitoring. A unit mask structure for focus monitoring has two patterns for position measurement and a light blocking film. The two patterns for position measurement are used to measure the mutual positional relationship formed on the surface of the substrate. The light blocking film is formed on the rear surface of the substrate and has a rear surface pattern that makes the incident directions of the exposure light that enters the two patterns for position measurement substantially differ. When the dimension of the rear surface pattern is L and the wavelength of the exposure light is xcex, L/xcex becomes 10 or greater.
According to the method of focus monitoring of the present invention, the dimension L of the rear surface pattern is determined so that L/xcex becomes 10 or greater, and, thereby, the dimension of the pattern can be enlarged to a degree such that the diffraction can be ignored. Due to the enlargement of the process dimension of rear surface pattern in the above manner, the fabrication of the photomask for focus monitoring is made easy.
In addition, the incident directions of the exposure light that enters the two patterns for position measurement are made to differ substantially by providing such a rear surface pattern and, thereby, a high detection sensitivity in the z direction, to the same degree as in the case wherein the "sgr" value is reduced in the conventional illumination, can be gained. In addition, since it is not necessary to reduce the "sgr" value, it is unnecessary to replace the illumination aperture unit at the time between focus monitoring and device pattern formation.
The above described method of focus monitoring is preferably provided with the step of applying a photoresist to the substrate as a photosensitive material, the step of exposing the applied photoresist to an image of the two patterns for position measurement of the photomask for focus monitoring, the step of forming a resist pattern by developing a patterning the exposed photoresist and the step of focus monitoring based on the mutual distance between the respective image patterns of the two patterns for position measurement that are transcribed to the resist pattern.
Focus monitoring can be carried out by measuring the mutual distance between the respective image patterns of the two patterns for position measurement in the above manner.
In the above described method of focus monitoring, the method of exposing the applied photoresist to an image of the two patterns for position measurement of the photomask for focus monitoring is preferably provided with the first exposure step of exposing a photoresist to an image of the two patterns for position measurement of the photomask for focus monitoring, the step of moving the substrate on which the photoresist is applied in the direction perpendicular to the direction of the optical axis and the second exposure step of exposing the photoresist to an image of the two patterns for position measurement of the photomask for focus monitoring. Either of the images of the two patterns for position measurement to which the photoresist is exposed in the second exposure step overlaps either of the images of the two patterns for position measurement to which the photoresist is exposed in the first exposure step.
In the case that the focus becomes misadjusted, either of the images of the two patterns for position measurement that are exposed in the first exposure step and either of the images of the two patterns for position measurement that are exposed in the second exposure step are arranged in the x-y plane so as to shift in mutually opposite directions. Therefore, focus monitoring can be carried out by measuring this shift amount in the x-y plane.
In the above described method of focus monitoring, each of the two patterns for position measurement is preferably either the inner box pattern or the outer box pattern of the box-in-box type mark.
The shift amount of each of the image patterns in the x-y plane can be easily measured by using the box-in-box type mark in such a manner.
In the above method of focus monitoring, the step of measuring the mutual distance between the respective image patterns of the two patterns for position measurement that are transcribed to the resist pattern is preferably carried out by using an overlap inspection apparatus for inspecting the positional shift of the overlap by processing the images of the two image patterns that have been read in.
Thereby, the positional shift can be measured with high precision.
In the above described method of focus monitoring, the step of measuring the mutual distance between the respective image patterns of the two patterns for position measurement transcribed to the resist pattern is preferably carried out by observing the positions of the two image patterns by using a scanning-type electron microscope.
Thereby, the positional shift can be easily measured.
In the above described method of focus monitoring, the two patterns for position measurement are preferably formed so that, at least, either of the image patterns of the two patterns for position measurement is readable by a pattern position detection mechanism integrally attached to the exposure unit.
Thereby, measurement becomes possible by means of a unit having a simple configuration.
In the above described method of focus monitoring, focus monitoring is preferably carried out by measuring the electrical resistance of the conductive layer, which is the lower layer of the resist pattern etched by using the resist pattern gained by developing the photoresist exposed through the first and second exposure steps as a mask.
Focus monitoring can be carried out by measuring the electrical characteristics in such a manner.
In the above described method of focus monitoring, error in the amount of movement of the substrate in the step of moving the substrate in the direction perpendicular to the direction of the optical axis is preferably measured. The error in the amount of movement is subtracted from the amount of positional shift between one of the images of the two patterns for position measurement that is exposed to the photoresist in the second exposure step and the other image of the two patterns for position measurement that is exposed to the photoresist in the first exposure step.
Thereby, the error in the amount of shift can be subtracted so that focus monitoring can be carried out with increased precision.
In the above described method of focus monitoring, the photomask for focus monitoring is preferably further provided with a mask structure for correcting the wafer position shift amount having two additional patterns for position measurement for measuring the relationship between the mutual positions formed on the surface of the substrate and a pattern formed on the light blocking film formed on the rear surface of the substrate for allowing the incident directions of the exposure light that enters the above additional two patterns for position measurement to become substantially equal. The above described two additional patterns for position measurement are exposed to the photoresist through the first and second exposure steps. An error in the amount of movement is measured from the amount of positional shift of the pattern formed by one of the above described two additional patterns for position measurement being exposed to the photoresist through the first exposure step relative to the pattern formed by the other of the above described two additional patterns for positional measurement being exposed to the photoresist through the second exposure step.
Thereby, the error in the amount of movement can be subtracted so that focus monitoring can be carried out with increased precision.
In the above described method of focus monitoring, patterns gained through exposing the photoresist to a plurality of shots by changing the amount of focus offset of the exposure unit are preferably used to find the relationship between the amount of positional shift of the pattern and the amount of focus shift in advance so that the amount of focus shift is found from the amount of positional shift of the measured pattern based on the above relationship.
The amount of focus shift can be corrected in such a manner.
A unit for focus monitoring of the present invention is a unit for focus monitoring used in focus monitoring for measuring the position of the exposed surface in the optical system in order to adjust the focus of the optical image vis-à-vis the exposed surface at the time of pattern exposure and is provided with a photomask for focus monitoring, an illumination optical system and a projection optical system. A pattern is formed on the photomask for focus monitoring. The illumination optical system is used to irradiate the photomask for focus monitoring with the exposure light. The projection optical system is used to project the image of the pattern of the photomask for focus monitoring onto the photosensitive body. The photomask for focus monitoring is provided with a substrate for allowing the exposure light to pass through and a unit mask structure for focus monitoring. The unit mask structure for focus monitoring has two patterns for position measurement and a light blocking film. The two patterns for position measurement are formed on the surface of the substrate and are used to measure the relationship between the mutual positions. The light blocking film is formed on the rear surface of the substrate and has the rear surface pattern for allowing the incident directions of the exposure light that enters the two patterns for position measurement to differ substantially. When the dimension of the rear surface pattern is L and the wavelength of the exposure light is xcex, L/xcex becomes 10 or greater.
According to unit for focus monitoring of the present invention, the dimension L of the rear surface pattern is determined so that L/xcex becomes 10 or greater, and, therefore, a pattern of large dimensions can be gained to the degree that diffraction can be ignored. The process dimensions of the rear surface pattern are increased in such a manner and, thereby, the fabrication of the photomask for focus monitoring becomes easy.
In addition, the incident directions of the exposure light that enters the two patterns for position measurement are substantially differentiated by provided such a rear surface pattern and, thereby, a high detection sensitivity in the z direction at approximately the same level as in the case where the "sgr" value is reduced using the conventional illumination can be gained. In addition, since it is not necessary to reduce the a value, it is unnecessary to replace the illumination aperture unit at the time between focus monitoring and device pattern formation.
A manufacturing method of an electronic device of the present invention is characterized by the use of a method of focus monitoring described in any of the above described aspects.
Thereby, focus monitoring becomes possible wherein the fabrication of a photomask is easy, the replacement of the illumination aperture unit is unnecessary and the detection sensitivity in the z direction is high and, thereby, patterns can be formed at a low cost and with a high precision.
In the above described manufacturing method of an electronic device, the device that is formed by using the above described method of focus monitoring is preferably a semiconductor device.
The above described manufacturing method of an electronic device is suitable for the manufacture of a device that is able to be manufactured by using a semiconductor manufacturing process such as a thin film magnetic film or a liquid crystal display element and is also suitable for the manufacture of a semiconductor device.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.